Dave,
I assume that the date stamp on a print file will be the date/time it was originally written to disk until another application opens it. Once this happens then the modified date/time will change.
Not sure whether it would work but have you tried checking for an OPEN file? You could run a test on the file to see if it is locked or open for write by another application and only access it yourself once the ‘print’ process has completed.
Regards
Garry